A VNA (Vector Network Analyzer) is an item of RF or microwave test equipment that characterizes the behavior of a path between two ports of a network. (Networks can have more than two ports, but the majority of networks of interest are of the two port variety. Techniques exist for combining a collection of two port characterizations to describe networks with more than two ports when such is necessary. Our present interest is with networks having two ports.) A two port network can be just about anything, ranging from a length of transmission line with connectors to an arrangement of components (e.g., an attenuator). The network might be passive (e.g., a resistive power splitter) or active (e.g., an amplifier). The usual VNA architecture has been devised with such linear networks in mind, and is for many applications generally quite satisfactory. It typically has a swept RF or microwave source that is applied first in a ‘forward’ direction to one end of the NUT (Network Under Test) while the amount and phase of applied power and reflected power are measured with calibrated narrowband receivers whose tuning tracks the frequency of the source. The power transmitted through the NUT in the forward direction is also measured by a similar receiver. The swept source is next applied to the NUT in the other (‘reverse’) direction, and similar measurements taken. The various receivers are all narrowband superheterodyne mechanisms using a single main LO (Local Oscillator) with mixers at their front ends to convert each of the four measured signals to a common IF (Intermediate Frequency). Each receiver measures both the amplitude and phase of its applied input signal. Since all the frequencies at the receivers are the same at any point in the sweep, the existence of a common reference plane will allow the measured phase values to be commensurate with one another, and the applied signal can be taken as the reference having 0° phase against which the phase values of the other signals are measured.
Associated with the measurements made by the receivers are four basic parameters customarily called a1, b1, a2 and b2. They are assumed to be voltage waves. By convention, the ports are subscripted as one and two, and, a is assumed to be the voltage incident upon or applied to a port, while b is the voltage emanating from or leaving a port. The ports are expected to be coupled to transmission lines of known characteristic impedance, say, 50 Ω. Recalling that power equals E2/R, and since a1 is a voltage, the square of a1 is proportional to the power entering the network at Port One while the square of b2 is proportional to the power leaving the network from Port Two. We shall follow the convention that the ‘FORWARD’ direction through the network is from Port One to Port Two, while ‘REVERSE’ is from Port Two to Port One. The narrowband superheterodyne receivers extract each of a1, b1, a2 and b2 as an amplitude and a phase. These complex values are the basis for an s-parameter measurement (‘s’ is for scattering) of s21, s12, s11, and s22 that can be understood and used on their own terms (as reflection and transmission coefficients), converted to other parameter types (e.g., Z, Y or H), or, converted to impedance and displayed as a function of applied frequency. Since measurements are taken in both forward and reverse directions, the results include all the transmission properties and the reflection properties of the NUT; which is to say, its complete characterization as a ‘black box’ with two ports.
As powerful as this usual manner of VNA operation is, it is not well suited for self-contained use with networks that are FTDs (Frequency Translation Devices), such as a mixer. In an FTD the frequency of the applied power at one port and the frequency of the power emerging from the other port are not the same. Typically, they will be separated by a fixed amount(s) determined by an auxiliary LO. If the NUT were a harmonic generator, then a more complicated case would obtain. The appearance of these frequencies that are different from that applied by the source means that they are not measured by the receivers (which are tracking the frequency of the source), with the consequence that the usual VNA architecture is not applicable to such networks. (One might also note that a mixer usually has three ports. On the other hand, it is usually not the case that a network path involving power flow through the LO port is of primary interest, and even if we supply an LO we can safely pretend that the LO for that mixer “is self-contained” and that the mixer is somehow a two port network that translates frequency.)
There have been several attempts to extend the architecture of the VNA to include the measurement of FTDs as two port networks. A first of these is the addition of a so-called ‘frequency offset’ mode of operation. This involves the inclusion of an auxiliary LO supplied to (or perhaps by) the NUT. In any event, the main LO is still arranged to operate in a phase and frequency locked loop that is driven by the untranslated applied frequency during times while a forward measurement is under way, so that a1 and b1 can be obtained as usual. Subsequent to that, but still during a forward measurement, the main LO is then driven by the translated frequency, which will allow the appropriate receiver to measure the magnitude (but not the phase) of b2. During times when a reverse measurement is being made (if such makes sense for the type of device being tested), a corresponding situation obtains that has the same general structure (but in the other direction) where a2 and b2 are valid, and b1 provides magnitude only. This circumstance arises because there is no way to provide a reference phase for b1 or for b2.
U.S. Pat. No. 6,064,694 discloses a method for characterizing FTDs that requires three mixers, an IF filter and three sets of measurements to obtain the s-parameters of a NUT under test that includes a mixer. However, errors caused by interactions owing to mismatches between the IF filter and the mixers are not removed from the measurement.
The techniques proposed by Joel P. Dunsmore in an article entitled “Novel Method for Vector Mixer Characterization and Mixer Test System Vector Error Correction” at pages 1833-35 of the MTT-S Digest, IMS2002, uses a pre-characterized mixer and filter pair to calibrate a VNA having a frequency offset mode of operation. It obtains relative phase and group delay characteristics of an FTD. However, the pre-characterized mixer must be reciprocal: RF to IF and IF to RF conversion must be the same, and the conversion loss must be small. The reverse characteristics cannot be measured, and an IF filter is required. The characteristics of that IF filter cannot be removed unless it has previously been separately characterized.
Joel P. Dunsmore and Michael E. Knox describe these and other related topics in two U.S. Pat. Nos. 6,448,786 B1 and 6,690,722 B1.
There is also an up-conversion/down-conversion technique that uses external mixing and filtering components to re-convert the IF from an FTD under test back to the frequency of the original RF stimulus. See, for example, FIGS. 4 and 24 in the Application Note 1287-7 entitled “Improving Network Measurements of Frequency-translating Devices” from Agilent Technologies, Inc. The test set-ups described there are very much do-it-yourself affairs (much has to be taken into account to be able to interpret the results), and do not lend themselves to reverse measurements.
Another disadvantage of several of these techniques is that they interpose ancillary items between the ports of the NUT and the test ports of the VNA. This raises various issued related to calibration and uncertainty, and generally makes the measurement more complicated.
What is needed is a way of improving the internal architecture of a VNA so that it can inherently make all the needed measurements on an FTD without the use of auxiliary paraphernalia and complicated external test set-ups.